Research And Design Of Capacitance Sensor And Measurement System In Dielectric Constant Measuring Instrument

The dielectric constant is mainly used to describe the important parameters of the response of the medium to the external electric field.It can be used as an index to verify the performance of the material and further solve some specific problems in production and life.Among many dielectric measurement methods,the capacitance method has low cost,good linearity and simple operation.It can realize contact and non-contact dielectric measurement according to the detection requirements.Aiming at the demand of portable dielectric measurement,this paper mainly introduces the principle analysis and structure design of capacitance sensor,capacitance detection circuit and system structure,and studies and designs three low-cost and practical dielectric constant measuring instruments.The main work and achievements of this paper are as follows :1)The dielectric constant measuring instrument proposed in this paper is mainly composed of capacitance sensor,capacitance detection circuit and microcomputer module.In order to facilitate the subsequent system design and physical construction,two capacitance sensors and capacitance detection circuits are proposed.Firstly,the parallel plate and interdigital structure capacitance sensor are designed.Based on the principle of the sensor,the small size capacitance sensor is modeled,simulated and analyzed in depth.For the parallel plate sensor,the equipotential protection ring is designed to reduce the edge effect of the plate,and the size of the sensor and the protection ring that can use the ideal parallel plate model is determined.For the interdigital sensor,the theoretical derivation determines that the interdigital capacitance is linearly related to the dielectric value to be measured,and the slotting scheme of the dielectric substrate between the two interdigital electrodes is given to ensure the stable operation of the detection circuit.Secondly,the capacitance frequency conversion circuit and capacitance voltage conversion circuit are designed.After analysis,the relationship between the output signal of the circuit and the sensing capacitance is clarified,and the improvement method of the detection circuit is given according to the characteristics of the sensor.Considering the whole assembly of the subsequent dielectric system,the sensor and detection circuit are designed to the same size with reference to the size of the microcomputer.2)A dielectric measuring instrument with parallel plate structure combined with capacitance frequency(CF)conversion circuit is designed by using parallel plate capacitance sensor and capacitance frequency conversion circuit.The sensor and circuit are fabricated by double-sided printed circuit board(PCB).For the measurement system,the equal potential is added to the capacitance frequency conversion circuit to ensure the effect of the sensor protection ring.After the system is built,the simplified dielectric constant model can be obtained by analyzing the dielectric system.After calibration and according to the insulation characteristics of the sample to be tested,two formulas for calculating the relative dielectric constant of the measuring instrument are given.Finally,the microcomputer can detect the output frequency of the system and display the relative dielectric constant of the sample to be tested.In order to test the performance of the parallel plate structure dielectric system,a variety of dielectric samples were measured and analyzed.The results show that the average relative error of the relative dielectric constant is within3.4%,and the measured dielectric value can be used as the reference dielectric value of the subsequent interdigital structure measuring instrument.3)The interdigital structure is designed to combine the capacitive frequency(CF)conversion circuit and the capacitive voltage(CV)conversion circuit dielectric measuring instrument.For the dielectric measuring instrument combined with CF conversion circuit,the analysis shows that the relative dielectric constant of the dielectric sample is proportional to the period of the output signal of the capacitor frequency conversion circuit,so only two standard samples are needed to complete the system calibration.The average relative error of the system is 4.5%.For the dielectric measuring instrument combined with CV conversion circuit,the complex dielectric constant of the sample can be measured due to the use of AC excitation capacitor voltage conversion circuit.The analysis shows that the sensing capacitance and the equivalent leakage conductance are related to the amplitude and phase of the output signal of the circuit,and the sensing capacitance and the leakage conductance are linearly related to the real and imaginary parts of the complex permittivity,respectively,and then the complex permittivity model of the system is obtained.The average relative error of the system is 7.8%.